The advent of the waveguide gas laser allows the fabrication of more compact lasers. Narrowing the bore of the discharge tube provides increased gain, power generated per unit volume and saturation parameter. This increase in power is achieved even though linear gas waveguides leak radiation into the walls of the dielectric because of the existence of well defined, low loss modes of laser propagation. The performance enhancement results from favorable de-excitation of the gas by wall collisions, device operation at increased gas pressure, and by reduction in gas temperature due to the improved thermal conduction provided by the waveguide walls.
In an effort to further increase output power, several improvements have been developed. One device presented by H. R. Schlossberg in U.S. Pat. No. 4,367,554 of Jan. 4, 1983 has increased output power by employing a plurality of hollow dielectric channels within a chamber containing a CO.sub.2 gas mixture. The diameter of the individual dielectric channels must be selected such that each channel is incapable of sustaining a guided mode of propagation, and be fabricated with an optically leaky dielectric. Only the multiplicity of leaky channels in combination provides sufficient gain for lasing within a device that comprises the totality of channels.
Another technique which has been used to increase the output power of gas lasers is to provide a mechanism for phase locking one laser relative to another, either by injection locking or optical coupling techniques. Phase locking maintains spatial coherence between the individual resonators.
Injection locking of adjacent lasers is accomplished by feeding the output of a single laser into a parallel array of optical resonators which act as amplifiers. If the lengths of the different optical paths are adjusted properly, the phase of the amplifier's output would be constant. Although this technique will provide phase locking and increased power, it mandates the use of external optical assemblies, such as mirrors and mounts, all of which must be precisely adjusted and which are subject to environmental degradation.
Another effective technique of providing phase locking is through optical coupling of adjacent lasers. Phase locked operation of adjacent lasers by optical coupling has been demonstrated in both waveguide gas lasers and semiconductor lasers. However, the physics which describe semiconductor lasers differ substantially from that which describes waveguide gas lasers. Semiconductor lasers are characterized by a guiding region whose index of refraction is greater than the surrounding cladding material. Waveguide gas lasers possess a guiding region having an index of refraction much less than the surrounding material.
Optical coupling in the two types of lasers occurs from two entirely different mechanisms. Semiconductor lasers couple as a result of the existence of an evanescent portion of the guided optical field in one resonator being present in another closely proximate resonator. Optical coupling between adjacent waveguide gas lasers cannot be by evanescent field coupling, but can only result from optical radiation loss or "leaks" between adjacent lasers. Consequently, techniques used to optically couple semiconductor lasers have limited applicability with waveguide gas lasers.
The optically coupled waveguide gas lasers of the prior art of one type comprise an elongated chamber that is divided into a plurality of longitudinal waveguides by using partitions made from an optically transmitting dielectric material. Prior art lasers of this type are excited by conventional DC or RF discharge that is provided to each optical resonator. The optically transmissive dielectric provides a lossy boundary through which energy leaks from one resonating cavity to the next, effectively coupling the phases and changing the amplitude distribution of the waveguide modes.
Coupled waveguide gas lasers of the prior art provide for increased power and phase locking. However, these devices are expensive to fabricate because of the dielectric array contained within the chamber. In addition, the amount of energy which "leaks" from one cavity to the next is limited to relatively small levels because of the high reflectivity of the transmitting material at the obligue angles of incidence which characterize mode propagation in waveguides. This in turn limits the operating parameters under which stable phase locked operation can be achieved.
Optically transmitting dielectric materials absorb power to some extent and reduce the overall amount of power available to the laser. It is well known in the art that employing a lossy dielectric separation between resonators will favor the "antisymmetric" phased locked normal mode of operation. This mode of operation is undesirable for most applications because the laser output beam possesses a power null across the optic axis. Moreover, the compartmentalized design of these lasers burdens them with poor thermal conductivity and undesirable gas flow characteristics. It is difficult to obtain optically transmitting materials in the infrared that are also good heat conductors for carrying away the heat generated by the electrical power dissipated in the laser's plasma column.
Initiating and sustaining a discharge in the resonator regions is difficult. With either DC or RF excitation the resonator ridge geometry produces a higher electric field strength within the gap above the ridges than in the resonators. Consequently, coupled ridge waveguide gas lasers of the prior art prevent gas breakdown in the gap and limit the discharge to the resonator regions. The additional apparatus needed to confine the discharge entails using additional components resulting in higher costs.